Zeolites, having molecular-sieve effects based on pores derived from the framework structures of the zeolites as well as properties such as ion exchange abilities, catalytic abilities, and adsorption abilities, have been currently widely used as adsorbents, ion exchange agents, industrial catalysts, and environmental catalysts.
For example, catalysts for exhaust gases, using zeolites on which a metal such as copper is supported, specifically, a CHA type aluminosilicate zeolite and a silicoaluminophosphate (SAPO) zeolite, have been developed. Notations such as the AEI type and the CHA type are codes that define the framework structures of zeolites, specified by the International Zeolite Association (IZA).
The AEI type zeolite has been known to have the same sizes of the pores of the zeolite as those of the CHA type zeolite but to have a structure having higher catalytic activity. An example in which the AEI type zeolite is used in a selective catalytic reduction (SCR) catalyst is detailed in Patent Literature 1. A catalyst with a low Si/Al ratio has been known to be able to be more preferably used when being used as an SCR catalyst for exhaust gas treatment from an automobile or the like, particularly in order to reliably perform treatment of an exhaust gas at the time of low-temperature operation such as the time of start. This is because in the case of an aluminosilicate-based zeolite, the coordination sites of transition metals and the like which become active sites are aluminum sites, and therefore, the catalyst with a low Si/Al ratio, having many active sites, is more advantageous.
A production method described in Patent Literature 2 is basic to common methods for producing an AEI type zeolite. In the specific production method in such a case, for example, DMDMPOH (N,N-dimethyl-3,5-dimethylpiperidinium hydroxide) as an organic structure-directing agent (SDA) is added to a Y type zeolite (framework density: 12.7 T/1000 Å3) and colloidal silica as raw materials, and the resultant is stirred in the presence of NaOH and subjected to hydrothermal synthesis for 8 days, thereby obtaining an AEI type zeolite. In this Patent Literature 2, it is described that the synthesis is possible even when aluminum sulfate, aluminum hydroxide, or the like is used, but experiments were actually conducted by using a Y type zeolite as a raw material and changing an SDA in substantially all Examples. In addition, it is current common technical knowledge that it is actually impossible to synthesize an AEI type zeolite when no Y type zeolite is used in the common production methods. This is corroborated by a description that “Although synthesis using Al(OH)3 as a raw material was attempted, no crystalline oxide was obtained” in for example, Non Patent Literature 1 or the like.
In Patent Literature 1, a Y type zeolite is also used as a raw material.
Patent Literature 3 discloses that an AEI type zeolite is obtained by a method for producing an AEI type zeolite with the use of inexpensive aluminum nitrate as a raw material, the method including mixing aluminum nitrate and tetraethylorthosilicate (TEOS) with SDA and then adding hydrofluoric acid. The method disclosed in Patent Literature 3 is a method for producing an AEI type zeolite having a Si/Al ratio of 200 or more, wherein the use of hydrofluoric acid prevents Al from being taken in zeolite as much as possible.
Patent Literature 4 described that an AEI type zeolite is obtained from aluminum oxide but does not describe that an AEI type zeolite was able to be actually produced in Examples and the like. In Patent Literature 4, there is used a production method, referred to as a dry gel conversion method, including treating a dry gel obtained by drying a raw material mixture for synthesizing a zeolite with steam or steam containing a volatile organic amine, thereby crystallizing a zeolite. In this method, it is still difficult to industrially mass-produce a homogeneous zeolite, and it has been desired to be able to perform production by a time-proven hydrothermal method.
Non Patent Literature 2 discloses a method for synthesizing an AEI type zeolite from a structure-directing agent containing phosphorus and a raw material containing a Y type zeolite. However, when a phosphorus compound is used as the structure-directing agent, hazardous diphosphorus pentaoxide may be generated by calcining a zeolite synthesized in order to remove the structure-directing agent, and a step is complicated in the case of removing phosphorus by treatment such as extraction. Therefore, the method using the structure-directing agent containing phosphorus is an industrially undesirable method.
Even when each method is used, a Si/Al ratio can be decreased merely to around 7. In Comparative Example 3 in Patent Literature 5, it is described that an AEI type zeolite having a Si/Al ratio of 5.5 was produced by using a Y type zeolite as a raw material; however, it was possible to obtain only the AEI type zeolite having extremely low crystallinity, e.g., having almost no activity after hydrothermal endurance treatment. Therefore, a production method capable of producing an AEI type zeolite having an arbitrary Si/Al ratio, particularly an AEI type zeolite having a lower Si/Al ratio, and a method for producing the AEI type zeolite have been demanded.